From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: blk-mq request allocation stalls [was: Re: [PATCH v3 0/8] dm: add request-based blk-mq support] Date: Fri, 09 Jan 2015 14:11:24 -0700 Message-ID: <54B043FC.8000902@kernel.dk> References: <20141224182143.GA12922@redhat.com> <20141224185529.GA13246@redhat.com> <20141224192643.GA30461@redhat.com> <54A6DB1D.4030201@acm.org> <20150105213557.GA5030@redhat.com> <54ABAB80.70006@acm.org> <20150106160553.GB10224@redhat.com> <54AC0A39.90801@kernel.dk> <54AD0B63.3010505@acm.org> <20150109194955.GA32641@redhat.com> <54B042FE.2000205@kernel.dk> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="------------000801010603060707090500" Return-path: In-Reply-To: <54B042FE.2000205@kernel.dk> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Mike Snitzer , Keith Busch , Bart Van Assche Cc: Christoph Hellwig , Jun'ichi Nomura , device-mapper development List-Id: dm-devel.ids This is a multi-part message in MIME format. --------------000801010603060707090500 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit On 01/09/2015 02:07 PM, Jens Axboe wrote: > On 01/09/2015 12:49 PM, Mike Snitzer wrote: >> On Wed, Jan 07 2015 at 3:40pm -0500, >> Keith Busch wrote: >> >>> On Wed, 7 Jan 2015, Bart Van Assche wrote: >>>> On 01/06/15 17:15, Jens Axboe wrote: >>>>> blk-mq request allocation is pretty much as optimized/fast as it >>>>> can be. >>>>> The slowdown must be due to one of two reasons: >>>>> >>>>> - A bug related to running out of requests, perhaps a missing queue >>>>> run >>>>> or something like that. >>>>> - A smaller number of available requests, due to the requested >>>>> queue depth. >>>>> >>>>> Looking at Barts results, it looks like it's usually fast, but >>>>> sometimes >>>>> very slow. That would seem to indicate it's option #1 above that is >>>>> the >>>>> issue. Bart, since this seems to wait for quite a bit, would it be >>>>> possible to cat the 'tags' file for that queue when it is stuck >>>>> like that? >>>> >>>> Hello Jens, >>>> >>>> Thanks for the assistance. Is this the output you were looking for >>> >>> I'm a little confused by the later comments given the below data. It >>> says >>> multipath_clone_and_map() is stuck at bt_get, but that doesn't block >>> unless there are no tags available. The tags should be coming from one >>> of dm-1's path queues, and I'm assuming these queues are provided by sdc >>> and sdd. All their tags are free, so that looks like a missing wake_up >>> when the queue idles. >> >> Like I said in an earlier email, I cannot reproduce Bart's hangs running >> mkfs.xfs against a multipath device that is built ontop of a virtio >> device in a KVM guest. >> >> But I can hit __bt_get() failures on the virtio-blk device that I'm >> using for the root device on this guest. Bart I'd be interested to see >> what you get when running the attached debug patch (likely will just >> echo the same type of info you've already provided). >> >> There does appear to be something weird going on with bt_get(). With >> the debug patch I'm seeing the following when I simply run "make install" >> of the kernel (it'll run dracut to build the initramfs, etc): >> >> You'll note that in all instances where __bt_get() returns -1 nr_free >> isn't 0. > > Yeah, that doesn't look good. Can you try with this patch? The second > hunk is the interesting bit, the first is more of a cleanup. Actually, try this one instead, it should be a bit more precise than the first. -- Jens Axboe --------------000801010603060707090500 Content-Type: text/x-patch; name="tag-v2.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="tag-v2.patch" diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 60c9d4a93fe4..2e38cd118c1d 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -143,7 +143,6 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx, static int __bt_get_word(struct blk_align_bitmap *bm, unsigned int last_tag) { int tag, org_last_tag, end; - bool wrap = last_tag != 0; org_last_tag = last_tag; end = bm->depth; @@ -155,15 +154,16 @@ restart: * We started with an offset, start from 0 to * exhaust the map. */ - if (wrap) { - wrap = false; + if (org_last_tag) { end = org_last_tag; - last_tag = 0; + last_tag = org_last_tag = 0; goto restart; } return -1; } last_tag = tag + 1; + if (last_tag >= bm->depth - 1) + last_tag = 0; } while (test_and_set_bit(tag, &bm->word)); return tag; @@ -199,9 +199,13 @@ static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt, goto done; } - last_tag = 0; - if (++index >= bt->map_nr) + index++; + last_tag = (index << bt->bits_per_word); + + if (index >= bt->map_nr) { index = 0; + last_tag = 0; + } } *tag_cache = 0; --------------000801010603060707090500 Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline --------------000801010603060707090500--